The present invention is directed to a method for the production of adiponitrile from acrylonitrile. In particular, the present invention is directed to a method of producing adiponitrile from acrylonitrile utilizing a first row transition metal catalyst such as cobalt.
Cobalt mediated acrylonitrile hydrodimerization to adiponitrile was reported in the literature in the late 1960's. The reaction was a two-step process broadly involving (1) mixing acrylonitrile, cobalt chloride and a reducing agent such as manganese powder in dimethylformamide and heating the solution and (2) adding a quenching reagent such as water or hydrogen sulfide to the solution. It was postulated that the first step involved the reduction of cobalt.sup.2+ to a lower oxidation state. This reduced cobalt metal then coordinated with two molecules of acrylonitrile, forming an intermediate complex. In the second step, the quenching (terminating) of the intermediate occurred with either water or hydrogen sulfide. The two-step process is best illustrated by the two equations appearing below: ##STR1##
As shown in the above equations, this method resulted in the desired dimer, adiponitrile (ADP), and either cobalt hydroxide or cobalt sulfide, depending on which quench agent was used. The disadvantage of this procedure is that these cobalt species are unreactive toward further acrylonitrile dimerization. That is, these reactions produce inactive cobalt species thereby limiting the reaction to one that, at best, yields only a stoichiometric amount of dimer when compared to cobalt.
Subsequent investigators attempted to study the cobalt metal mediated acrylonitrile dimerization process in more detail. Alternate reducing agents (manganese, magnesium, or zinc), solvents (dimethylformamide, acetonitrile, ethanol, or benzene) and quenching reagents were tested. In all cases the selectivity to adiponitrile was generally high (over 90%), however, the turnover number (# molecule ADP produced/# molecules cobalt used) never went above 1. That is, the reaction was stoichiometric in the cobalt (one molecule) used compared to the amount of adiponitrile produced (one molecule). Finally, investigators discovered that the slow addition of a quench solution comprising an ammonium chloride/methanol solution produced a system which was catalytic. That is, the turnover number (ADP/cobalt) increased from 1 to the range of about 3.5 to 4.5 with an adiponitrile selectivity in the range of 30 percent. As can be readily seen, the selectivity to adiponitrile was lowered dramatically when the reaction was made catalytic to cobalt.
In summary, the linear hydrodimerization of acrylonitrile to adiponitrile by cobalt-based systems has been reported in the literature. Moreover, the reaction can be made catalytic in cobalt by slow addition of a quench solution comprising ammonium chloride and methanol. This quench method has resulted in ADP/cobalt turnover numbers of approximately 4 with selectivity to ADP of about 30 percent. The present invention is directed to the development of a cobalt promoted dimerization process for the production of adiponitrile from acrylonitrile which substantially improves the ADP/cobalt turnover number while simultaneously increasing the selectivity of the reaction to adiponitrile.